The Tarnaev brothers were cruelly successful, but they are far from the only terrorists over the past decade with big ideas about carnage in America. There is a temptation with each act of terror to see it as an isolated act, connected to the mental state of the actor, but not to larger forces. The FBI used to have theories about "Sudden Jihad Syndrome" and "Lone Wolves" that were not only wrong, but also pulled law enforcement off the track.

"Sudden Jihad Syndrome" was invented by the FBI to explain why people who lived quietly in the United States for some period of time "suddenly" went berserk and killed others. Why Naveed Haq shot six people at the Seattle Jewish Federation, killing one; why Hesham Hadayet, an Egyptian with a history of radical statements, shot up the El Al ticket counter in LA; why Derek Shareef, a convert to Islam, planned to firebomb a mall in Rockford, IL; and why Bosnian Sulejman Talovic, killed five people in a shopping center in Salt Lake City. It was supposed to explain why Abdulhakim Mujahid Muhammad shot up an Army recruiting center in Little Rock, and Kosovar Arid Uka killed two soldiers on a U.S. Army bus in Germany. The FBI won't call it "jihad," but a "bolt from the blue" was also supposed to explain why Maj. Nidal Hassan killed 13 people at Ft. Hood; it doesn't explain why he yelled "Allahu Akbar" as he did it.

According to the FBI, they were all "normal" guys until they weren't; none were "affiliated" with terror organizations. The official version was that they were under some "unknown" mental stress that made them "snap" one day. Happily for Americans, what the FBI says and what it does can be two different things.

While sifting and learning from prior experiences, the FBI stopped the Ft. Dix pizza truck bombers and the Lackawanna Six before anyone was killed. In 2006 there was a foiled plan to flood lower Manhattan with bombs either in the Holland Tunnel or the PATH train tunnel under the World Trade Center site that would bring down the walls that hold back the river. In June 2007, seven men were arrested in a plot against the Sears Tower in Chicago and a Federal building in Miami. The would-be bombers were looking for al Qaeda connections, but found the FBI. Also in 2007, groups of men including Iraqis and Pakistanis were arrested in the Midwest and West Virginia. They had purchased more than 1,000 disposable cell phones from various stores -- untraceable and used as bomb detonators in Madrid in 2004.

The FBI stopped talking about "Sudden Jihad Syndrome" and began talking about "Lone Wolves," sick individuals, they said, who were still not connected -- not literally connected -- to anyone or anything that could be blamed. Well, not until 2011 Times Square bomber Faisal Shahzad, a 31-year-old former budget analyst from Connecticut (with a wife and child, which was supposed to make him unlikely to be a terrorist), acknowledged in court that he had been trained and paid by the Pakistani Taliban. He wasn't told when exactly to do the deed, but he was indeed "connected."

There is no unconnected terrorism.

In 2007, NYPD studied 11 cases of homegrown jihadists, isolating specific factors that appear to move some people -- primarily young men -- to radical, violent activity even as most American Muslims remain unmoved by or even repulsed by the idea of violence committed in the name of religion. The findings of Radicalization in the West: The Homegrown Threat, were summarized at the time:

• Salafist ideology combines Islam with a determination to solve problems through violence. Salafist institutions and literature are readily available in the West. 

• Al Qaeda provides inspiration, but generally not operational assistance. 

• Susceptible people seek an identity or a cause and often self-identify before finding compatriots. Radicalization has proceeded more slowly in the U.S. than in Europe, where even second and third generation immigrants have trouble assimilating into the local culture -- but more quickly since 9-11. 

• The Internet is an enabler, providing an anonymous virtual meeting place. Sites other than mosques can provide the sense of community otherwise isolated people may be seeking. 

• A "spiritual sanctioner" and an "operational leader" are necessary to move people from the ideological phase to an operational terrorist cell. 

• Not everyone who begins the process of radicalization becomes a terrorist; there are several points at which people drop out.

For the unremarkable conclusion that professional jihadists use the Internet to find susceptible people with self-identity problems seeking causes, and that even those people often drop out of the process at several points, the NYPD was called "racist" and "Islamophobic."

But it should be no surprise that people who listen to and venerate radical ideologies -- in this case, radical Islam -- read jihadist material, travel to places where jihad is taught and learned may be influenced and radicalized by jihad. People who are uncomfortable and unhappy in their present lives can be influenced by the promise of the benefits of jihad and may be moved to make it happen. Nidal Hassan had shown public evidence of his strong interest in jihad and was corresponding with Anwar al-Awlaki.

Dzhokhar and TamerlanTsarnaev were from mostly Sunni, independence-minded and often-jihadist southern Russia. They had been in the U.S. for some period of time and Dzhokhar appears to have received a small scholarship from the City of Cambridge when he graduated from high school. His Facebook page indicates some trouble finding his place in the United States.

We're going to learn more about them both, but what we know about the process of radicalization and the connection between unhappy people, true believers, and criminal terrorist enablers is the beginning of wisdom -- if we choose to see it.

Shoshana Bryen is Senior Director of The Jewish Policy Center